Synthesis of Pd Core-Pt Shell Structured Catalyst By a Very Simple Direct Displacement Reaction

Abstract

Introduction Carbon supported Pd core-Pt shell structured catalyst (Pt/Pd/C) is a promising candidate for decrease of Pt usage in PEFCs owing to its high Pt utilization and oxygen reduction reaction (ORR) activity [1, 2]. Previously, we synthesized Pt/Pd/C catalyst by a modified Cu-UPD/Pt displacement method [3], and also developed an electrochemical high activation protocol (HAP) and a H2-O2 chemical treatment to enhance ORR activity of the catalyst [4]. Besides ORR activity enhancement, synthetic method suitable for a mass production of the catalyst is highly demanded. In this study, we synthesized the Pt/Pd/C catalyst by a very simple direct displacement reaction (DDR) in which a carbon supported Pd core (Pd/C) was directly displaced with a Pt precursor. We believe that DDR is a much simpler synthetic method compared with the modified Cu-UPD/Pt displacement one and suitable for mass production of the catalyst. Features and electrochemical properties of Pt/Pd/C catalyst synthesized by DDR were investigated. Experimental Pt/Pd/C catalyst was synthesized by DDR. 300 mg of Pd/C core (mean size: 4.4 nm, metal loading: 32 wt.%, ISHIFUKU Metal Industry) was dispersed in 200 mL of water and pH was adjusted to 1 by 2 M H2SO4 at 5°C under N2 gas atmosphere. K2PtCl4 corresponding to Pt monolayer shell was added and stirred at 5°C for 0.5 h, followed by stirring at 70°C for 3 h, by which Pt/Pd/C catalyst was synthesized. Pt/Pd/C catalyst was activated by HAP (rectangular potential cycling of 0.05 V (300 s)-1.0 V (300 s) vs. RHE performed in Ar saturated 0.1 M HClO4 at 80°C for 50 cycles [4]) and by H2-O2 chemical treatment (H2 and O2 gasses were alternately introduced in 2 M H2SO4 containing Pt/Pd/C catalyst at 80°C, in which equilibrium potentials of H2 (ca. 0.00 V) and O2 (ca. 0.98 V) were alternately applied to the catalyst [4]). Durability of Pt/Pd/C catalyst was evaluated by an accelerated durability test ADT (rectangular potential cycling of 0.6 V (3 s)-1.0 V (3 s) vs. RHE performed in Ar saturated 0.1 M HClO4 at 80°C for 10,000 cycles). Pt/Pd/C catalyst was characterized by TG, XRD, XRF, TEM, TEM-EDX and CV measurements. ORR activity of Pt/Pd/C catalyst was evaluated by RDE technique performed in O2 saturated 0.1 M HClO4 at 25°C with a positive scan rate of 10 mV/s at 1,600 rpm. Results and Discussion Irregular Pt shell formation on Pd core observed in Pt/Pd/C catalyst synthesized by a modified Cu-UPD/Pt displacement method was suppressed in Pt/Pd/C catalyst synthesized by DDR as demonstrated in TEM images in Fig. 1. CVs of Pd/C core and Pt/Pd/C catalysts are summarized in Fig. 2 with focusing on anodic scan in a potential range of 0.05-0.5 V. Large hydrogen desorption wave observed in Pd/C core was much reduced in Pt/Pd/C catalyst synthesized by DDR compared with catalyst synthesized by Cu-UPD method, indicating that Pt shell coverage was increased by DDR. It is considered that potential difference of Pd core and [PtCl4]2- in DDR was decreased compared with that of Cu shell and [PtCl4]2- in Cu-UPD method, which slowed down speed of displacement reaction and suppressed irregular Pt shell formation, leading to increase of Pt shell coverage. Furthermore, fine catalyst particles disappeared and particle density decreased, which improved durability of the Pt/Pd/C catalyst. ORR mass activity of Pt/Pd/C catalysts is summarized in Fig. 3. Initial ORR mass activity of Pt/Pd/C catalyst synthesized by DDR exhibited 1,028 A/g-Pt and the activity was enhanced to 1,685 A/g-Pt by HAP, which was 5.3-fold of a reference Pt/C catalyst (320 A/g-Pt, TEC10E50E, TKK). The observed high ORR mass activity is considered to arise from the high Pt shell coverage of Pt/Pd/C catalyst synthesized by DDR. Activation of Pt/Pd/C catalyst by H2-O2 chemical treatment and a single cell performance will be presented in the meeting. Acknowledgement This study was supported by NEDO, Japan.